In general, asynchronous (pronounced ay-SIHN-kro-nuhs, from Greek asyn-, meaning "not with," and chronos, meaning "time") is an adjective describing objects or events that are not coordinated in time. In information technology, the term has several different usages.
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1) In telecommunication signaling within a network or between networks, an asynchronous signal is one that is transmitted at a different clock rate than another signal. (Plesiochronous signals are almost but not quite in synchronization - and a method is used to adjust them - and synchronous signals are those that run at the same clock rate.
2) In computer programs, asynchronous operation means that a process operates independently of other processes, whereas synchronous operation means that the process runs only as a result of some other process being completed or handing off operation. A typical activity that might use a synchronous protocol would be a transmission of files from one point to another. As each transmission is received, a response is returned indicating success or the need to resend. Each successive transmission of data requires a response to the previous transmission before a new one can be initiated.
Synchronous program communication is contrasted with asynchronous program communication.